1. Field of the Invention
The present invention relates to an olefin type thermoplastic elastomer powder for powder slush molding, a powder slush molding method using the same, and a molded article thereof. More specifically, it relates to a thermoplastic elastomer powder for powder slush molding which is suitable for use, mainly, as covering materials and housings for molded articles in various fields, irrespective of backing materials, a powder slush molding method using the same and a molded article thereof.
2. Description of the Related Art
With regard to covering materials used as interior decorative materials for automobiles, there is an increasing demand for those which have a light weight and a good and soft feeling, and which can be given an embossed pattern or stitch pattern to increase the value of molded articles. Also, during the scrapping of cars, acidic substances are generated by a combustion of such interior decorative materials to cause a contamination of the air, and thus create serious social problems such as acid rain, etc., and therefore, there is a strong demand for non-polluting materials.
Known covering materials of the prior art are vacuum molded olefin type thermoplastic elastomer (hereinafter called TPO) sheets and vacuum formed sheets composed mainly of vinyl chloride resins and ABS resins, or rotational molded or slush molded sols composed mainly of vinyl chloride resins for paste and plasticizers (hereinafter called sol moldings).
Vacuum formed TPO sheets attain the objects to provide a light weight and non-polluting material, but it is difficult to impart complicated shapes thereto.
Also, vacuum formed sheets contain a high residual molding stress caused during the forming process, and thus have a drawback in that cracks will appear after long term usage.
Vacuum formed sheets composed mainly of vinyl chloride resins and ABS resins have drawbacks similar to those of vacuum formed TPO sheets in that it is difficult to impart complicated shapes thereto, and further that, compared with TPO, they have the drawback of heavier weight and cause pollution.
Sol moldings composed mainly of vinyl chloride resins for paste and plasticizers give a soft feeling, and complicated shapes can be imparted thereto. However, since the gellation temperature is low, they melt rapidly while being molded, and thus many drawbacks arise, for example, problems in the processing such as flow marks, lip or a sol fiber-forming phenomenon, the inherent problems of vinyl chloride such as a heavier weight and pollution, and further problems in that the inner window glass surface of automobiles produced from said sol molding suffers from hazy appearance due to the use of a large amount of plasticizers.
Due to these drawbacks and problems of the molding method, the powder slush molding method has recently attracted attention.
Powder molding methods include, in general, a flow dipping method, an electrostatic coating method, a flame spray coating method, a powder rotational molding method, and a powder slush molding method, and particularly for an interior decorative materials for automobiles, the powder slush molding method is most suitable.
A partially crosslinked TPO composition is known from Japanese Unexamined Patent Publications (Kokai) Nos. 48-26838, 53-149240, but the known molding methods currently used for the partially crosslinked TPO are:
a. injection molding (shear rate during processing: .gamma..gtoreq.10.sup.3 sec.sup.-1); PA1 b. extrusion molding (10.sup.1 .ltoreq..gamma..ltoreq.10.sup.2 sec.sup.-1); PA1 c. calendering (10.sup.2 .ltoreq..gamma..ltoreq.10.sup.3 sec.sup.-1); PA1 d. compression molding of the primarily processed product in b. or c.; and PA1 e. vacuum forming of the primarily processed product in b. or c., PA1 "1) An embossed thermoplastic elastomer molding, having an emboss pattern formed on the surface produced by spraying of a thermoplastic elastomer powder comprising a polyolefin resin and an ethylene-.alpha.-olefin copolymer rubber onto the inner surface of a heated mold with an emboss pattern, thereby melting and adhering said powder to the inner surface of said mold. PA1 2) A method of preparing an embossed thermoplastic elastomer molding having an emboss pattern on the surface according to a slush molding method, which comprises spraying a thermoplastic elastomer powder comprising a polyolefin resin and an ethylene-.alpha.-olefin copolymer rubber onto the inner surface of a previously heated mold with an emboss pattern, thereby melting and adhering said powder to the inner surface of said mold, and then cooling said mold."
but all of these methods require the molding temperatures are higher or equal to the softening point, and the molding pressures must be varied depending on the viscosities and the shear rates corresponding to the respective processing conditions.
Nevertheless, in the molding method such as a powder slush molding at a shear rate of 10.sup.0 sec.sup.-1 or less or under an approximately stationary state of the polymer, at a vibration frequency of 1 radian/sec., and with no application of a pressure or under a very low pressure (.ltoreq.1 kg/cm.sup.2), the flowability becomes extremely poor, and accordingly, the molding becomes very difficult. Further, even if molding is possible, the poor flowability in the low shear rate region cause an incomplete thermal fusion between powder particles, and only molded articles with a low mechanical strength can be obtained.
For this reason, for example, Japanese Unexamined Patent Publication (Kokai) No. 2-57310 discloses the following inventions:
Thus, there is an example in which an improvement is effected by melting and then adhering TPO with a low flowability in a low shear rate region to the inner surface of the previously heated mold by a spray gun.
According to this method, however, because the powder is fed and adhered to the inner portion of the mold, problems arise in that the sheet thickness of the molding becomes nonuniform, and that pinholes may be formed.
Also, because the powder is sprayed onto a previously heated mold in the open state without being close contact with the powder feeding box and adhered to the inner surface of the mold, the problems arise of a scattering of the powder outside and an entrainment of foreign matter from outside, and a molding fully satisfying the appearance, physical properties and another requirements has not been obtained.